1. Field of the Invention
The present invention relates generally to a circuit board on which various semiconductor devices or electronic components are to be mounted. Particularly, the present invention relates to a resin board (a thermally conductive board) with high heat dissipation that is suitable for use in the power electronics field and to a power module including the same.
2. Related Background Art
Recently, there have been demands for increases in density and improvements in function of semiconductors, as a higher performance and a smaller size are required in electronic equipment. Accordingly, smaller higher-density circuit boards on which such semiconductors are to be mounted also have been required. Hence, a design with consideration to heat dissipation of a circuit board has become important. As a technique of improving heat dissipation of a circuit board, a method employing an insulated metal substrate using a metal plate made of, for example, copper or aluminum instead of a conventional printed board formed of glass-epoxy resin has been known in which a circuit pattern is formed on one surface or both surfaces of the metal plate with an insulating layer interposed therebetween. When higher thermal conductivity is required, a board has been used in which a copper sheet is bonded directly to a ceramic board made of, for example, alumina or aluminum nitride. Generally, an insulated metal substrate is utilized for use with a relatively low power consumption. In this case, the insulated metal substrate is required to have a thin insulating layer to have an improved thermal conductivity. Accordingly, the insulated metal substrate has a problem in that a portion between the circuit pattern and the metal plate as a ground tends to be affected by noise easily, and also has a problem in withstand voltage.
In order to solve such problems, recently, a board has been proposed in which a composition containing a filler with high thermal conductivity added to a resin is combined with a lead frame as an electrode to form one body. For example, JP 10(1998)-173097 A proposes a board including such a composition. A method of manufacturing such a thermally conductive board is shown in FIGS. 15A and 15B. According to JP 10(1998)-173097 A, a mixed slurry containing at least an inorganic filler and thermosetting resin is used to form a film and thus a sheet-like thermally conductive resin composition 16 is produced. The composition 16 is dried and then is superposed with a lead frame 11 and a heat sink 13 as shown in FIG. 15A. Next, as shown in FIG. 15B, they are heated and compressed and the composition 16 is cured to form an insulating layer 12. Thus, a thermally conductive board 28 is produced.
In such a thermally conductive board and a power module including the same, generally a part of the wiring pattern is grounded to the heat sink according to electrical requirements such as reducing stray capacitance. In such a case, it is necessary to connect the wiring pattern and the heat sink outside the board and an additional step other than steps for component mounting is required. FIG. 16 shows an example of the connection with the heat sink in such a case. According to this example, a part of the lead frame 11 is used as a grounding pattern 15 and a lead 29 is soldered to the grounding pattern 15 and the heat sink 13 to establish a ground connection. In the figure, numeral 12 indicates an insulating layer. In order to connect the wiring pattern and the heat sink outside the board, it also is necessary to place the grounding pattern at the periphery of the board. This is a constraint in pattern design, and the degree of freedom in circuit design has been decreased accordingly. Furthermore, there has been a problem in that the size of the board itself increases to allow the establishment of the connection.